Nonpolypoid (or “flat”) lesions (NPLs) represent a subset of colonic neoplasms whose prevalence, clinical significance and detectability by CT colonography have been and still are a source of debate and concern. NPLs were first described by Japanese physicians who reported detecting them in a high number of patients undergoing colonoscopy (as summarized in [ 1]), as opposed to European and American endoscopists who were reporting NPLs with a much lower prevalence [ 2].

Reasons for this discrepancy are multiple. First of all, there are differences in operators' skills: Japanese endoscopists have been trained for a long time in the identification of early gastric cancer and they therefore applied their experience to colonic lesions. In fact, it has been recently demonstrated that proficiency in the diagnosis of NPLs improves the detection rate [ 3]. Second, the availability and extensive use of high-end technology (magnification, chromoendoscopy, narrow-band imaging), first developed in Japan, can also explain the better performance of Japanese clinicians [ 4]. Third, the definition of “flat” lesion is not univocal, leading to consequent problems of misclassification. The clearest example comes from the prospective National Polyp Study [ 5] in which no flat lesions were reported. In 2004, a retrospective reclassification of over 1500 lesions that had been generically classified as “polypoid” in that study led to 474 of them (27% of all baseline adenomas) being reclassified as “flat” [ 6]. Fourth, a further problem still incompletely solved, is the debate among pathologists about the definition of “flat polyp” which is another source of confusion [ 7].

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